Dendritic polymers (or “dendrimers”) are polymers with hyperbranched structures which can comprise a high number of reactive functional groups exposed at the peripheral edges of the hyperbranched dendrimer molecule. Depending on the degree of branching, dendritic polymers may be classified into first, second, third, fourth or even fifth generation dendritic polymers. A first generation dendritic polymer may theoretically have a total of eight peripheral reactive functional groups, whereas a second generation polymer will have theoretically sixteen peripheral functional groups, and whereas a third generation polymer will have theoretically thirty two peripheral functional groups and so forth. The total number of peripheral functional groups per molecule is also referred to as the peripheral functionality.
Dendritic polymers have been used in the field of manufacturing protective coatings due to its unique structure which leads to the formation of high performance coatings. For example, the molecules of dendritic polymers mimic the hydrodynamic volumes of spheres, and as such, they can be used to provide coatings of high molecular weights whilst maintaining relatively low viscosity. At the same time, dendritic polymers provide coatings with high crosslink density at the same time keep its flexibility.
Conventionally, protective coatings comprising dendritic polymers are provided as solvent-based coating systems due to the dendritic polymer's lack of solubility in water. Such solvent-based systems provide excellent abrasion resistance, flexibility, adhesion, and chemical resistance. However, due to the presence of organic solvents which are volatile in nature, coatings prepared from solvent-based systems will typically emit an undesirably high level of volatile organic compounds (“VOC”). In recent years, ever stricter regulatory requirements in many countries have driven coating manufacturers to explore the possibilities of non solvent-based coating systems.
Accordingly, water-based coating systems have been proposed to overcome the problem of VOC emission. However, conventional water-based coating systems have poorer properties in terms of hardness and chemical resistance than solvent borne coating systems. In one study, dendritic polymers whose peripheral reactive groups have been functionalized with ionic functional groups (also termed “ionomers”) were proposed. In this study, the proposed ionomers were dispersed in water and comprised an approximately 40% solid content. However, such water-based coating systems suffer from some technical drawbacks. For instance, when these ionomers are mixed with cross-linkers in one-component (“1K”) or two-component (“2K”) preparations to form water-based coatings, there is a tendency for the formed coating to experience phase separation which may be due to the reactions between cross-linkers and the water solvent.
To address this problem, it has been suggested to add excess surfactant to these water-based coating systems in a bid to prevent phase separation. While the addition of excess surfactant does ameliorate the problem of phase separation, it results in an overall softened coating when applied to a surface. This can be highly undesirable for applications such as protective coatings where the surface hardness of the coating is one of the key properties required for the coating to fulfill its protective function.
In addition to the above, water-based coatings comprising the above mentioned ionomers also suffer from poor homogeneity. As a result′, the applied coatings typically have uneven surfaces and exhibit undesirable blistering, resulting in the coated article having a poor aesthetical appearance. Additionally, the reaction rate between cross-linkers and the ionomers have been observed to be less than satisfactory. In particular, it has been postulated that due to the high specific surface area of the dendritic polymer, the peripheral reactive groups are situated very closely to each other. Such proximity can cause a significant amount of steric hindrance, which in turn impedes the reaction rate between the ionomer and cross-linkers.
Therefore, there is a need to provide a water-dispersible coating that overcomes, or at least ameliorates, the technical problems above. In particular, there is a need to provide a water-dispersible coating that does not experience phase separation, displays a high level of homogeneity, has excellent film forming properties, readily reacts with cross-linkers and is capable of being rapidly cured after being applied onto a surface. There is also a need to provide a method for producing such a water-dispersible coating.